AIP's Physics News Highlights: Feb. 12, 2012

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Physics News Highlights of the American Institute of Physics (AIP) contains summaries of interesting research from the AIP journals, notices of upcoming meetings, and other information from the AIP Member Societies. Copies of papers are available to journalists upon request.

TOPICS IN THIS ISSUE:1. Virtual Ghost Imaging: New technique enables imaging even through highly adverse conditions: By using some of light’s “spooky” quantum properties, researchers have created images of objects that might otherwise be hidden from view.2. New ‘Soft’ Motor Made from Artificial Muscles: The electrostatic motor, used more than 200 years ago by Benjamin Franklin to rotisserie a turkey, is making a comeback in a promising new design for motors that is light, soft, and operates without external electronic controllers.3. Boiling Breakthrough: Nano-coating doubles rate of heat transfer: The old saw that a watched pot never boils may not apply to pots given an ultra-thin layer of aluminum oxide (alumina), which researchers have reported can double the heat transfer from a hot surface to a liquid. 4. Other Content: Upcoming Conferences of Interest; AIP Science Communication Awards: Call for Entries Deadline Feb. 17._______________________________________________________1. Virtual Ghost Imaging: New technique enables imaging even through highly adverse conditions

Ghost imaging (GI), and its even more oddly named cousin virtual ghost imaging (VGI), seem to contradict conventional wisdom by being able to image an object by simply counting photons in a “light bucket.” This non-intuitive technique, however, can lead to better images when conditions are less than ideal. In a first-of-its-kind demonstration, a team of researchers from the U.S. Army Research Laboratory in Adelphi, Md., and the University of Maryland in Baltimore, captured reflected photons from a highly specialized laser beam to create a VGI image of a remote target.

In the case of VGI, reflection does not refer to a mirror image of an object. Rather it is merely the individual reflected photons of light that are counted with a single-pixel camera known as a light bucket.

“Virtual ghost imaging is an amazing tool,” says Ronald Meyers, a quantum physicist with the U.S. Army Research Laboratory, in a paper published in the American Institute of Physics’ journal Applied Physics Letters. “Because we are no longer bound by the need to collect spatial information – as is necessary in a typical camera – we can produce an image in some rather adverse and highly obscured conditions.”

In normal ghost imaging, harnessing information to make an image is a two-step process. First, you analyze the light source, which could be the sun or a lamp, with a charge-coupled device (CCD) camera. You then use a second detector, a light bucket, to count the reflected photons. By combining the data from the light source with the properties of the collected photons, an image can be created.

The trick to making an image from photons that contain no spatial information lies in physics related to “entanglement,” a property of light that Einstein referred to as “spooky action at a distance.” Through entanglement, photons (individual packets of light) can share a certain degree of information. This property is already being developed for specialized communications and computers.

Virtual ghost imaging is a more self-contained and robust application of this phenomenon. For example, in VGI, one light source was a laser that produced an incredibly coherent beam of light known as a Bessel beam. Bessel beams, unlike normal laser beams, produce concentric-circle patterns. If a portion of the beam is blocked or obscured along its trajectory, the original pattern eventually reforms. “Bessel beams are self-healing and provide an important tool in virtual ghost imaging,” said Meyers. “Even after passing through distortions or a mask, the same well-defined ring shapes reemerge.” So long as enough photons make it to the target and back to the single-photon detector, it’s possible to construct an image.

In their proof-of-concept demonstration, the researchers compared a Bessel beam’s VGI imaging capabilities with that of a normal “Gaussian” laser beam. Their target was the letters “ARL.” The light was then reflected back to the single pixel bucket detector. The researchers conducted this same test several times, placing different objects or an obscuring medium in the paths of the two light beams. In each case – whether passing through an offset aperture, cloudy water, or heat distortion – the Bessel beam reformed to produce a recognizable VGI image. The Gaussian beam produced a much less faithful image, and, in the case of the offset aperture, produced virtually no image at all.

"What this demonstrates is that by combining virtual ghost imaging with a highly diffraction-free coherent light source like a Bessel beam, it's possible to probe through conditions that would normally thwart other imaging technologies," Meyers says.

According to the researchers, potential spin-offs of ghost imaging and virtual ghost imaging include applications in Intelligence-Surveillance-Reconnaissance (ISR), medical imaging, and quantum computing.

Article: “Virtual Ghost Imaging through Turbulence and Obscurants using Bessel Beam Illumination” is published in Applied Physics Letters.

"Perhaps the earliest public demonstration of an electric motor," writes a team of researchers from the University of Auckland in New Zealand, "involved the automatic rotation of a turkey on a spit over a fire" at a party put on by Benjamin Franklin in 1749. Franklin's electrostatic motor was self-commutating, meaning that it was able to provide a continuous torque while it turned without requiring external electronics to control its progress. Using artificial muscles, hyper-elastic materials that expand when a charge is applied, the New Zealand team has made a prototype for a self-commutating artificial muscle motor that does not require external electronics or hard metal parts. The researchers describe the device in a paper accepted to the American Institute of Physics’ journal Applied Physics Letters.

The team's proof-of-concept motor is controlled with carbon-based switches whose resistances change when they are compressed, which activates artificial muscles that rotate a shaft. The artificial muscles, in turn, are able to activate the switches by their movements. All that is required to operate the device is a direct current input voltage. Among the advantages of these electrostatic motors compared to their harder, bulkier electromagnetic cousins, the authors write, is that they are capable of delivering higher torque, require low currents instead of high, and can have a flatter profile. The new motor in its current state is inefficient, but the authors hope their prototype will open the door to a softer, lighter future for electrostatic motors, with applications in areas such as prosthetics and soft robots – applications well beyond “simply barbecuing poultry."

By adding an incredibly thin coating of alumina to a metal surface, researchers at the Georgia Institute of Technology have doubled the rate that heat travels from a solid surface – such as a pot on a stove – into the liquid in the pot. The results are published in the American Institute of Physics’ journal Applied Physics Letters. Pool boiling is the most common and familiar method of heating a container’s contents, and is a remarkably efficient heat transfer method. The transfer of heat in this case is referred to as the “heat flux.” There exists, however, a critical point at which a solid surface gets too hot and pool-boiling efficiency is lost. “Delaying the critical flux could play an important role in advancing thermal management of electronics as well as improving the efficiency of a number of energy systems,” says Bo Feng, Ph.D., the Georgia Tech researcher leading this project. In boiling, bubbles carry away large amounts of heat from solid surfaces, but the bubbles also act as an insulator, preventing the liquid from rewetting the surface and thereby interrupting heat transfer. The alumina coating – only a few hundreds of atoms thick (1/1,000 the thickness of a human hair) – has a high affinity to water and, as a result, facilitates the rapid rewetting of the solid surface. “This is the primary reason for the enhancement of heat transfer,” says Feng. An atomic layer deposition technique was used to control the thickness. By achieving such a thin coating, the additional layer of alumina did not appreciably increase thermal resistance, but it did increase the overall heat transfer. “The potential contribution of this investigation lies in tailoring the wettability of surfaces at the nanometer scale, thereby greatly increasing the heat transfer during pool boiling,” adds G.P. “Bud” Peterson, Ph.D., director of Georgia Tech’s Two-Phase Heat Transfer Lab. “This is especially promising for applications where the implementation of nanotube or nanowire arrays are possible.” Nanotube and nanowire arrays are another effective way to enhance pool boiling heat transfer. Combining these two techniques – nanotube and/or nanowire arrays and nano-coating by atomic layer deposition – may increase pool-boiling efficiency even further.

Article: “Enhancement of Critical Heat Flux in Pool Boiling Using Atomic Layer Deposition of Alumina” is published in Applied Physics Letters.

AIP Science Communication Awards: Final Call for EntriesEntries are requested for the American Institute of Physics’ 2012 Science Communication Awards, which recognize effective science communication, both in print and new media, that improves the general public's appreciation of physics, astronomy, and allied science fields.

More information and an entry form are available at http://www.aip.org/aip/writing.To receive Physics News Highlights by email, please contact Catherine Meyers at cmeyers@aip.org.

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